The present invention relates to a refrigerating system employing a screw compressor and, more particularly, to a refrigerating system having a function of quickly controlling the capacity in response to a change in the refrigeration load. In general, the screw compressor for refrigerating purpose is equiped with a hydraulically operated slide valve. The capacity of the compressor can be controlled linearly and continuously by a suitable control of the slide valve.
Such a linear and continuous capacity control of the screw compressor by the slide valve is shown in, for example, Japanese Utility Model Publication No. 4564/1977. More specifically, this literature shows a screw compressor having a slide valve, which has additionally a suction block type capacity controller provided at the suction side thereof to widen a controllable range of the capacity of the compressor. In operation, when there is a drastic reduction in the load, the suction block type capacity controller having quick response characteristics responds first to the reduction and then the slide valve is operated, so that the capacity of the compressor can be varied over a wide range. In the refrigerators such as chilling units, however, there is a demand for compressors having a simpler capacity controller capable of quickly responding to a change in the load linearly and continuously.
Generally, the capacity control in refrigerators such as chilling units in response to the load demand is conducted in the following manner. Namely, the set temperature of a chilled water temperature controller or warmed water temperature controller is periodically compared with the outlet water temperature of an indoor heat exchanger, and when any difference between the set temperature and the outlet water temperature exists, the slide valve of the compressor is operated for a fixed predetermined time. Since the duration of operation of the slide valve is fixed regardless of the magnitude of temperature difference, it is often experienced that the capacity control fails in adequately responding to the load demand due to insufficiency of the control time, particularly when the difference between the set temperature and the water temperature is large.